The present invention relates generally to digital signal detector arrangements and in particular to a dynamic DC offset compensation arrangement as well as a Viterbi equalizer with integrated dynamic DC offset compensation in wireless systems.
Wireless technology provides a plurality of applications for voice and/or data transmission. Today's cell phone networks offer a plurality of services for their customers including digital data services, such as digital email, Internet access, etc. In future applications, such as third generation wireless networks, a plurality of new digital data services will be provided. In particular, Internet applications will be highly improved and made more practical, for example, via high speed digital data transmission. Other digital data application, not yet applicable in today's wireless transmission technology, will be adapted and implemented.
High speed wireless data applications require high data throughput at a significantly lower bit error rate than voice applications. Bit errors in voice applications are usually easy to recover or do not need to be fully recovered due to redundancy capabilities of the human ear; whereas, digital data applications often highly rely on the correctness of the submitted data. The quality of data transmissions in a digital environment highly depends on the quality of the transmission channel. Under severe channel conditions, the mobile device throughput is markedly affected due to retransmission of erroneous data packets, thus affecting the entire network throughput. This situation may be ameliorated by the use of antenna diversity and more sophisticated signal processing algorithms.
According to the prior art, decision feedback equalizers are used to compensate for the effects of the transmission channel, which can vary depending on the environment. A basic decision feedback equalizer (DFE) consists of a forward filter, a feedback filter, and a decision device. Decision feedback equalizers are effective against severe intersymbol-interference. Intersymbol-interference is an effect which creates distortion of the transmitted signal in a specific way. In a sequence of positive and negative symbol pulses, intersymbol-interference is the distortion of a symbol pulse within a particular symbol period caused by the smearing or spillover of symbol pulses of preceding and/or succeeding adjacent symbol pulses into the particular symbol period. The spillover of the preceding and/or succeeding symbol pulses will add to or subtract from the symbol pulse in the particular symbol interval depending upon whether the adjacent interfering symbol pulses are positive or negative in value. In applications with mobile devices, intersymbol-interference occurs due to the multi-path profile of the mobile channel as well as the above mentioned smearing which is generated due to analog filtering. Unlike linear equalizers, decision feedback equalizer's decision errors propagate in the feedback branch thus affecting the outcome of future bit decisions.
In digital communication receivers, an important problem is that of estimating the channel impulse response in the presence of a DC offset. In communication systems using frequency hopping, this needs to be done without knowledge about previous bursts. If the DC offset is constant throughout the burst, several techniques exist to remove the offset and subsequently perform the channel impulse response estimation. One way is, for example, to simply average or least square circular fit the signal. Another option is the joint detection of DC and the channel impulse response, which seems to work particularly well for non-constant envelope signals.
Dynamic DC signals are often experienced in the receiver with a Direct Conversion Radio Frequency (DC-RF) receiver architecture. For example, an input signal to such a DC-RF receiver may be given as:x(t)=A cos ωt  (1)
The output of a square non-linearity is then given as:
                              y          ⁡                      (            t            )                          =                                            α              2                        ⁢                                          x                2                            ⁡                              (                t                )                                              =                                                    1                2                            ⁢                              α                2                            ⁢                              A                2                                      +                                          1                2                            ⁢                              α                2                            ⁢                              A                2                            ⁢              cos              ⁢                                                          ⁢              2              ⁢                                                          ⁢              ω              ⁢                                                          ⁢              t                                                          (        2        )            
These non-linearities are usually found as imperfection in a DC-RF receiver, such as transistor mismatch in the signal path, oscillator signal leaking and self-down-converting to DC through the mixer, etc. FIG. 1 shows a scenario in which the desired baseband signal is located at DC plus an un-modulated interferer at the frequency ω. In this case, the power of the in-band interferer at DC is thus given as
      1    2    ⁢      α    2    ⁢            A      2        .  
If such a scenario takes place during a burst as shown in FIG. 2, conventional DC estimation will fail and the subsequent channel impulse response estimation will lead to a corrupted channel impulse response, causing equalization to fail. In FIG. 2, a typical burst data structure is shown as used, for example, in a GSM wireless environment. Such a burst consists typically of a left and a right data sequence comprising an intermediate test sequence TS. On top of the data burst structure a typical DC offset is schematically shown. In this example, the DC offset starts within the test data sequence and ends within the right data sequence. Other scenarios are possible.
Based on GSM 05.05, the interference level for an AM supression test is −31 dbm, and a typical AM supression of a RF chip is around 80 dB. This gives the peak energy of the DC offset at −31−80=−111 dbm, which is on the same level as the noise. Thus, it is very difficult to detect such a DC signal, because the DC offset itself is masked within the noise as both have similar power levels.